There are circumstances where transfer of images from a carrier to a media so as to produce a visible image requires multiple passes of the copy receiving media past one or more image transfer stations. The combining of two images from an image source or sources onto a single receiving sheet frequently demands repetitive movement of the sheet past the work station. For instance, one application is where prestored information defining a form is combined with data to complete that form. Another application is the transfer of color images which requires accurate recycling of the media through the transfer station to create a composite of different color elements such as from ribbons or ink sources. In the case of copiers and printers using electrophotographic or xerographic processes, different charge patterns are applied from the photoconductor to the copy sheet with intervening applications of different color toners correlating to the particular color image transferred.
Thus devices have evolved for recycling the image receiving copy media past an image transfer station. An example is U.S. Pat. No. 4,517,591 by Nagashima et al wherein the copy sheet is held on the peripheral surface of a drum by leading and trailing edge grippers. Such a system may function satisfactorily as long as the copy sheets are all of the same length.
An attempt to handle variable length sheets is suggested in U.S. Pat. No. 4,595,279 by Kuru et al which controls the speed of the sheet drive elements associate with the drum and the fuser. An arrangement for securing variable length copy sheets to a transfer drum is shown in Great Britain Pat. No. 2,181,415 which includes moveable rollers and suction cup grippers on the surface of the transfer drum. It requires physical intervention to set these components once the anticipated copy sheet size dimensions are known.
Another device for handling duplex copy transfer in the environment of a duplicator machine is shown in U.S. Pat. No. 4,443,094 by Ricciardi. An impression cylinder has an edge gripper and a continuously recycling chain with dual grippers cooperating to allow a first side imaging of the copy sheet after which the chain grippers strip the sheet from the cylinder and reverse it so the cylinder grips the trailing edge of the flipped sheet. To handle copy sheets of different lengths, it is necessary to physically relocate the grippers on the chain as well as to adjust the timing of the machine operation.
Accordingly, the prior art can accommodate variable sized copy sheets only by either physical relocation of sheet handling elements or by utilization of recycling paper path elements that establish a sufficiently long paper path to contain the longest sheet anticipated for use in the image transfer process. Unfortunately, such long paper paths penalize the machine in that its maximum throughput is tied to the length of the longest copy sheet the machine might encounter.